1. Field of the Invention
The field of the invention relates to improved methods of measurement to quantitatively determine the amount of oil and gas present from within a cased well disposed in geological formation in the earth. The field of the further invention relates to methods of measurement that may be used from within an old cased well in an existing oil reservoir; from within an old cased well in an existing gas reservoir; from within steel pipe attached to a drilling bit during drilling operations in a formation; or from within a new casing or pipe inserted into the earth for any reason.
2. Description of Prior Art
The oil industry has long sought to measure resistivity through casing. Such resistivity measurements, and measurements of other electrochemical phenomena, are useful for at least the following purposes: locating bypassed oil and gas; reservoir evaluation; monitoring water floods; measuring quantitative saturations; cement evaluation; permeability measurements; and measurements through a drill string attached to a drilling bit. Therefore, measurements of resistivity and other electrochemical phenomena through metallic pipes, and steel pipes in particular, are an important subject in the oil industry. Many U.S. patents have issued in the pertinent Subclass 368 of Class 324 of the United States Patent and Trademark Office which address this subject. The following presents a brief description of the particularly relevant prior art presented in the order of descending relative importance.
U.S. patents which have already issued to the inventor in this field are listed as follows: U.S. Pat. No. 4,820,989 (Ser. No. 06/927,115); U.S. Pat. No. 4,882,542 (Ser. No. 07/089,697); U.S. Pat. No. 5,043,668 (Ser. No. 07/435,273); U.S. Pat. No. 5,043,669 (Ser. No. 07/438,268); U.S. Pat. No. 5,075,626 (Ser. No. 07/434,886); U.S. Pat. No. 5,187,440 (Ser. No. 07/749,136); and U.S. Pat. No. 5,223,794 (Ser. No. 07/754,96). These seven U.S. Patents are collectively identified as "the Vail Patents" herein.
The apparatus and methods of operation herein disclosed are embodiments of the Through Casing Resistivity Tool.TM. that is abbreviated TCRT.RTM.. The Through Casing Resistivity Tool.TM. and TCRT.RTM. are Trademarks of ParaMagnetic Logging, Inc. in the United States Patent and Trademark Office. ParaMagnetic Logging, Inc. has its principal place of business located at 18730 - 142nd Avenue N.E., Woodinville, Wash., 98072, USA, having the following telephone number: (206) 481-5474.
An important paper concerning the Through Casing Resistivity Tool was published recently. Please refer to the article entitled "Formation Resistivity Measurements Through Metal Casing", having authors of W. B. Vail, S. T. Momii of ParaMagnetic Logging, Inc., R. Woodhouse of Petroleum and Earth Science Consulting, M. Alberty and R. C. A. Peveraro of BP Exploration, and J. D. Klein of ARCO Exploration and Production Technology which appeared as Paper "F", Volume I, in the Transactions of the SPWLA Thirty-Fourth Annual Logging Symposium, Calgary, Alberta, Canada, Jun. 13-16, 1993, sponsored by The Society of Professional Well Log Analysts, Inc. of Houston, Tex. and the Canadian Well Logging Society of Calgary, Alberta, Canada (13 pages of text and 8 additional figures). Experimental results are presented therein which confirm that the apparatus and methods disclosed in Ser. No. 07/434,886 that is U.S. Pat. No. 5,075,626 actually work in practice to measure the resistivity of geological formations adjacent to cased wells. To the author's knowledge, the SPWLA paper presents the first accurate measurements of resistivity obtained from within cased wells using any previous experimental apparatus.
Other recent articles appearing in various publications concerning the Through Casing Resistivity Tool and/or the Vail Patents include the following: (A) in an article entitled "Electrical Logging: State-of-the-Art" by Robert Maute of the Mobil Research and Development Corporation, in The Log Analyst, Vol. 33, No. 3, May-June 1992 page 212-213; and (B) in an article entitled "Through Casing Resistivity Tool Set for Permian Use" in Improved Recovery Week, Volume 1, No. 32, Sep. 28, 1992.
The inventor made a verbal presentation entitled "Introduction to the Through Casing Resistivity Tool" to the Permian Basin Well Logging Society on Mar. 18, 1993 that first publicly presented a portion of the inventive material herein.
The Vail Patents describe the various embodiments of the Through Casing Resistivity Tool ("TCRT"). Many of these Vail Patents describe embodiments of apparatus having three or more spaced apart voltage measurement electrodes which engage the interior of the casing, and which also have calibration means to calibrate for thickness variations of the casing and for errors in the placements of the voltage measurement electrodes.
U.S. Pat. No. 4,796,186 which issued on Jan. 3, 1989 to Alexander A. Kaufman entitled "Conductivity Determination in a Formation Having a Cased Well" also describes apparatus having three or more spaced apart voltage measurement electrodes which engage the interior of the casing and which also have calibration means to calibrate for thickness variations in the casing and for errors in the placements of the electrodes. This patent has been assigned to, and is owned by, ParaMagnetic Logging, Inc. of Woodinville, Wash. In general, different methods of operation and analysis are described in the Kaufman Patent compared to the Vail Patents cited above.
U.S. Pat. No. 4,837,518 which issued on Jun. 6, 1989 to Michael F. Gard, John E. E. Kingman, and James D. Klein, assigned to the Atlantic Richfield Company, entitled "Method and Apparatus for Measuring the Electrical Resistivity of Geologic Formations Through Metal Drill Pipe or Casing", predominantly describes two voltage measurement electrodes and several other current introducing electrodes disposed vertically within a cased well which electrically engage the wall of the casing, henceforth referenced as "Gard (518)". However, that patent does not describe an apparatus having three spaced apart voltage measurement electrodes and associated electronics which takes the voltage differential between two pairs of the three spaced apart voltage measurement electrodes to directly measure electronic properties adjacent to formations. Nor does Gard (518) describe an apparatus having at least three spaced apart voltage measurement electrodes wherein the voltage drops across adjacent pairs of the spaced apart voltage measurement electrodes are simultaneously measured to directly measure electronic properties adjacent to formations. Therefore, the Card (518) does not describe the methods and apparatus disclosed in the Vail Patents.
USSR Patent No. 56,026, which issued on Nov. 30, 1939 to L. M. Alpin, henceforth called "Alpin (026)", which is entitled "Process of the Electrical Measurement of Well Casings", describes an apparatus which has three spaced apart voltage measurement electrodes which positively engage the interior of the casing. However, Alpin (026) does not have suitable calibration means to calibrate for thickness variations of the casing nor for errors related to the placements of the voltage measurement electrodes. Therefore, Alpin (026) does not describe the methods and apparatus disclosed in the Vail Patents.
French Patent No. 2,207,278 having a "Date of Deposit" of Nov. 20, 1972 describes apparatus having four spaced apart voltage measurement electrodes which engage the interior of borehole casing respectively defined as electrodes M, N, K, and L. Various uphole and downhole current introducing electrodes are described. Apparatus and methods of operation are provided that determines the average resistance between electrodes M and L. French Patent No. 2,207,278 further explicitly assumes an exponential current flow along the casing. Voltage measurements across pair MN and KL are then used to infer certain geological parameters from the assumed exponential current flow along the casing. However, French Patent No. 2,207,278 does not teach measuring a first casing resistance between electrodes MN, does not teach measuring a second casing resistance between electrodes NK, and does not teach measuring a third casing resistance between electrodes KL. Various preferred embodiments described in the Vail Patents teach that it is of importance to measure said first, second, and third resistances to compensate current leakage measurements for casing thickness variations and for errors in placements of the voltage measurement electrodes along the casing to provide accurate measurements of current leakage into formation. Further, many embodiments of the Vail Patents do not require any assumption of the form of current flow along the casing to measure current leakage into formation. Therefore, for these reasons alone, French Patent No. 2,207,278 does not describe the methods and apparatus disclosed herein. There are many other differences between various embodiments of the Vail Patents and French Patent No. 2,207,278 which are described in great detail in the Statement of Prior Art for Ser. No. 07/754,965 dated Dec. 2, 1991 that issued as U.S. Pat. No. 5,223,794 on Jun. 29, 1993.
An abstract of an article entitled "Effectiveness of Resistivity Logging of Cased Wells by A Six-Electrode Tool." by N. V. Mamedov was referenced in TULSA ABSTRACTS as follows: "IZV.VYSSH.UCHEB, ZAVEDENII, NEFT GAZ no.7, pp. 11-15, July 1987. (ISSN 0445-0108; 5 refs; in Russian)", hereinafter the "Mamedov (1987)". It is the applicant's understanding from an English translation of that Mamedov (1987) that the article itself mathematically predicts the sensitivity of the type tool described in the above defined French Patent No. 2,207,278. Mamedov (1987) states that the tool described in French Patent No. 2,207,278 will only be show a "weak dependence" on the resistivity of rock adjacent to the cased well. By contrast, many embodiments of the Vail Patents, and the invention herein, provide measurements of leakage current and other parameters which are strongly dependent upon the resistivity of the rock adjacent to the cased well. Therefore, Mamedov (1987) does not describe the methods of measurement described herein.
U.S. Pat. No. 2,729,784, issued on Jan. 3, 1956 having the title of "Method and Apparatus for Electric Well Logging", and U.S. Pat. No. 2,891,215, issued on Jun. 16, 1959 having the title of "Method and Apparatus for Electric Well Logging", both of which issued in the name of Robert E. Fearon, henceforth called the "Fearon Patents", describe apparatus also having two pairs of voltage measurement electrodes which engage the interior of the casing. However, an attempt is made in the Fearon Patents to produce a "virtual electrode" on the casing in an attempt to measure leakage current into formation which provides for methods and apparatus which are unrelated to the Kaufman and Vail Patents cited above. The Fearon Patents neither provide calibration means, nor do they provide methods similar to those described in either the Kaufman Patent or the Vail Patents, to calibrate for thickness variations and errors in the placements of the electrodes. Therefore, the Fearon Patents do not describe the methods and apparatus disclosed herein.